Electronic protection devices such as voltage surge protectors are commonly used to protect electric or electronic equipment such as PLCs, computers, and entire electrical installations against destructive overvoltage surges. Such surge protection devices guard the electronic circuitry against detrimental power surges generated from various sources, including, but not limited to: motors, transformers, welding machines, lightning strikes, and power-grid-switching by the energy supplier. To protect against unacceptable voltage surges, voltage sensitive devices are employed to absorb or shunt current safely away from a circuit to be protected.
A very useful voltage sensitive device is a varistor such as a metal oxide varistor (MOV). MOVs are solid-state surge protective devices widely used with low-voltage AC circuits to protect electrical devices and sensitive loads. Varistors are non-linear electronic devices generally comprised of a ceramic compound for example, zinc oxide (ZnO) granules doped with other compounds—principally oxides of bismuth, cobalt, manganese, chromium, and tin. The material is fabricated by mixing finely powdered constituents of a binder agent. This mixture is pressed into thin disks and then fired in an oxidizing atmosphere at around 1200° C. The two faces of the disks are then coated with an electrically conducting compound and terminals are attached by soldering. The assembly is then coated with a thin covering of epoxy or other insulating material to provide electrical insulation and mechanical protection.
At nominal power system levels, a varistor presents a high resistance to a circuit and does not conduct any significant current. However, in a transient power surge condition, the varistor can be utilized to limit the transient over-voltage and to divert transient current surges away from the circuits to be protected. The effect of the varistor can be scaled to handle larger surge currents and energies by increasing the size of the varistor or by connecting multiple varistors in parallel. A varistor can be designed to limit transient voltages in circuits to be protected to a specified level can also be designed and configured to divert transient currents of specified current levels and/or wave shapes.
A chief characteristic of a varistor is that over a wide range of electrical current, the voltage drop across the varistor remains within a narrow band commonly called the varistor voltage. A log-log plot of the instantaneous voltage (in volts) versus the instantaneous current (in amps) yields a nearly horizontal line. Their current-voltage characteristics make varistors well suited for protection of sensitive electronic circuits against electrical surges, over-voltages, faults, and shorts. When subjected to a voltage exceeding its voltage limit, the varistor becomes highly conductive, absorbs and dissipates the energy related to the over-voltage, and typically limits the current to a neutral line or ground plane.
One significant limitation of a varistor is that during a power surge when a varistor is conducting high currents, it will generate heat in excess of what it can satisfactorily dissipate. The heat is generally proportional to the area of the varistor as well as the wave shape of the current and is a limiting factor in the capability of the varistor to conduct current. If an over-voltage condition is not timely discontinued, the varistor can continue to increase in temperature and can ultimately fail, i.e., rupture or explode. It is possible for such a failure to destroy nearby electronic components and equipment. The failure of a varistor in a surge suppression system may allow the fault condition to reach the sensitive electronic equipment the system was designed to protect.
Others have provided structures to prevent or ameliorate the over heating problems discussed above. For example U.S. Pat. No. 6,430,019 issued to Martenson et. al. discloses a “thermal switch” which physically disconnects electrical connection of the voltage sensitive device from its circuit upon an over-voltage thermal event. However, the structures disclosed in Martenson et. al. require a number and type of components, and arrangement of those components, that would appear to complicate construction and operation of the circuit protection device.
Thus, there presently is a need for a reliable and compact mechanism to prevent thermally related failures of circuit protection devices.
The present invention is provided to address these needs and to provide other advantages.